The present invention relates to a gas venting arrangement in an injection molding apparatus, and to a method for venting gas in the injection molding apparatus.
In an injection molding method such as a diecasting method, a molded product often contains voids in its interior due to injection of a molten metal into a mold cavity in which gases are extant. The gases are mingled with the molten metal and remain intact.
According to a conventional gas venting arrangement and gas venting method in the injection molding apparatus, a gas vent passage extending from the mold cavity is formed in mold halves, and a valve is disposed at the gas vent passage. With the structure, if molten material is injected into the mold cavity, while the valve is left open, gas in the mold cavity is discharged out of the mold halves.
In such type of arrangement and method for gas vent, the valve is closed by the flow-mode of molten material having sufficient injection velocity. If the molten material is injected at low velocity, the valve is incapable of closing. Further, in the conventional arrangement, molten material passes through a bypass passage until the moving energy of the molten material closes the valve. However, if valve closing timing is not stabilized, molten material may be inadvertently discharged through the gas vent passage out of the metal molds, and insufficient gas venting results.
Furthermore, various injection modes should have to be considered. For example, during injection of the molten material into the mold cavity, a part of the molten material may be dispersed into the gas vent passage as small molten grains. Alternatively, after the mold cavity is completely filled with the molten material, surplus molten material is continuously or discontinuously advanced into the gas vent passage.
According to the conventional gas venting arrangement, molten material may be discharged through the valve toward atmosphere or may be adhered to the valve, since the mode of the molten material cannot be promptly detected by mere detection of inertia force or pressure of the molten material or by the employment of a heat sensor. Therefore, insufficient gas venting results due to adhesion of molten material to the valve. Moreover, in the conventional arrangement, an inner diameter of the gas vent passage must be small in size so as to restrain prompt passing of the molten material therethrough for avoiding leakage of the molten material. Therefore, the gas venting function is also restrained by the small diameter passage.
Japanese Utility Model Publication No. 60-19806 discloses a gas venting arrangement in an injecting molding apparatus in which a gas vent control valve is closed by hydraulic pressure in response to an operation of an injection cylinder for injection a molten material into a mold cavity. As is well known, since the injection cylinder is positioned upstream with respect to the mold cavity, and since the gas vent control valve is disposed downstream thereof, the above-described drawbacks may not be overcome by mere detection of the movement of the injection cylinder.
Japanese Patent Application Publication (kokai) No. 61-17349 discloses a sensor mechanism which detects molten material reaching to a gate portion of the mold halves. This detection leads to operation of a vacuum valve mechanism. However, the sensor mechanism is disposed at the gate portion which portion is positioned upstream of the mold cavity. Therefore, this structure cannot attain prompt closing of the gas vent valve in response to the various modes of the injection molten material.
Japanese Patent Publication No. 41-10612 discloses a flange portion integral with a rod of a casting sleeve. When casting of the melted material is performed, the plunger rod is moved to push the melted material toward a mold cavity. During the rod movement, the flange portion abuts a switch so that a gas vent control valve is moved to close the vent passage, and vacuum valve is closed. With the arrangement, the vent control valve is inadvertently moved to close the vent passage in the injection period into the mold cavity. Therefore, resultant casted product may contain voids therein due to insufficient gas discharge. Further, a timer is used to render the internal space of the casting sleeve sufficiently filled with the melted product prior to the movement of the plunger rod. Therefore, the resultant structure becomes much more complicated.
The same is true with respect to Japanese Utility Model Publication No. 56-14923 and Japanese Patent Application Publication (kokai) No. 61-165262. In the former publication, a sensor is disposed at an upstream side with respect to the mold cavity to operably move the gas vent control valve. And in the latter publication, a sensor is disposed in a mold cavity to detect the melted material in order to operate the gas vent control valve in response to the detection. Therefore, in both publications, insufficient gas venting may occur, to thereby provide void containing casted product.
U.S. Pat. No. 4,431,047 discloses a gasventing arrangement incorporated with a mold which belongs to the technical field relevant to that of the present invention. Further, co-pending U.S. patent application, commonly assigned, has been filed bearing Ser. No. 26,317 filed Mar. 16, 1987, now U.S. Pat. No. 4,722,385.